Wet-type paper friction member

ABSTRACT

[Problems]To provide a wet paper friction member with both excellent friction property and high frictional coefficient. [Solution Means]A wet paper friction member is produced by impregnating a paper body with thermosetting resin, curing it by heating and then forming it by pressure. The paper body comprises cellulose pulp and a filler and further comprises aramid pulp the drainability of which is adjusted to 200-400 ml and aramid pulp the drainability of which is adjusted to 450-700 ml, respectively at 10-30 wt % of a total weight of the paper body, and said whole aramid pulp the drainability of which is adjusted comprises 30-50 wt % of a total weight of the paper body. Its surface roughness before and after pressure forming is adjusted so that the load length ratio Rmr (30%)≧55% as the load length ratio is defined by Rmr(C) (C: cutting level %). Thus, high frictional coefficient, heat resistance and mechanical strength are realized simultaneously.

[DETAILED DESCRIPTION OF THE INVENTION]

1. [Technical Field of the Invention]

The present invention relates to a wet or wet-type paper friction member used in a clutch and brake component of an automatic transmission of a vehicle.

2. [Prior Art]

A wet paper friction member is produced by dispersing a mixture of a fiber base material such as pulp and a friction modifier etc. in water and forming a paper body; making it dry and thereafter, impregnating it with thermosetting resin; curing it by heating; and then forming it by pressure. The functions of a wet paper friction member are influenced by a blend of the fiber base material, a filler and the friction modifier etc. which are main components of the paper body, and a method of surface finishing. Therefore, choice and blending amount of materials and methods of surface finishing have been researched for improvement of a wet paper friction member according to its use.

In particular, it has been known that fiber ingredient has an important role in having an influence on porosity and flexibility of a friction member and thus having an influence on its friction property and mechanical strength and that a friction modifier also affects porosity and surface configuration of the friction member.

For example, Patent Document 1 discloses a wet paper friction member characterized in that a paper body comprises cellulose pulp, a filler, and aramid pulp the drainability of which is adjusted to 250-440 ml and the amount of which is 30-80 wt % of the total weight of pulp, and also comprises a friction modifier containing diatomaceous earth the pipe section of which takes a shape of a netted cylinder with average length of 4-6 micrometers.

This wet paper friction member achieves relatively superior friction property and compression fatigue strength by specifying the range of blending ratio of pulp fiber and the kind of diatomaceous earth in a friction modifier.

[Patent Document 1]

Unexamined Japanese Patent Publication No. 026837/2000

[Problems to be solved by the Invention]

In a clutch and brake component of an automatic transmission, the function of transmitting torque is referred to as torque capacity, and is the most fundamental function for its design. It is desirable that torque capacity is high, but, when the frictional coefficient of a wet paper friction member is increased in order to achieve higher torque capacity, other functions of a clutch and brake component such as heat resistance and mechanical strength often tends to be deteriorated. Thus, a wet paper friction member which is excellent in torque capacity, heat resistance, and mechanical strength could not be easily produced.

While the wet paper friction member of Patent Document 1 has relatively excellent torque capacity, heat resistance, and mechanical strength to a certain extent, it was not completely satisfactory.

In view of the above problems, it is an object of the present invention to provide a wet paper friction member which is used mainly in a clutch and a brake of an automatic transmission and has both excellent friction property and high frictional coefficient. Improvement-was made by focusing attention on pulp and diatomaceous earth in a friction modifier forming a paper body among compositions of a conventional wet paper friction member and its surface roughness. This successfully enables to provide a wet paper friction member which has excellent friction property, high frictional coefficient, heat resistance, and mechanical strength simultaneously, and has high versatility without limitation for its use.

[Means to solve the Problems]

The present invention solved the above-mentioned problems by providing a wet paper friction member which is produced by impregnating a paper body with thermosetting resin, curing it by heating and then forming it by pressure, characterized in that said paper body comprises cellulose pulp and a filler and further comprises aramid pulp the drainability of which is adjusted to 200-400 ml and aramid pulp the drainability of which is adjusted to 450-700 ml, respectively at 10-30 wt % of a total weight of the paper body, and said whole aramid pulp the drainability of which is adjusted comprises 30-50 wt % of a total weight of the paper body.

[Function]

The present invention can provide a wet paper friction member which is excellent in torque capacity, heat resistance and mechanical strength, since a paper body comprises cellulose pulp and a filler and further comprises aramid pulp the drainability of which is adjusted to 200-400 ml, and aramid pulp the drainability of which is adjusted to 450-700 ml, respectively at 30-50 wt % of a total weight of the paper body, and said whole aramid pulp the drainability of which is adjusted comprises 30-50% of the total weight of the paper body.

[EMBODIMENTS]

A wet paper friction member according to the present invention is produced by blending a mixture of cellulose pulp, a filler, aramid pulp the drainability of which is adjusted and a friction modifier of specified shape and thus producing a paper body; impregnating it with thermosetting resin; curing it by heating; and then forming it by pressure. The paper body comprises two kinds of aramid pulp with the adjusted drainability of 200-400 ml and 450-700 ml, respectively at 10-30 wt % of the total weight of the paper body. The whole amount of aramid pulp the drainability of which is adjusted is 30-50% of the total weight of the paper body, and moreover, surface roughness is adjusted by a surface finishing process. Thereby, high frictional coefficient, heat resistance and mechanical strength are realized simultaneously.

Drainability of aramid pulp which forms said paper body is adjusted. The term “drainability” herein defined is meant by an index to indicate a degree of being drainable of the pulp and to indicate a degree of having been stricken of the fiber. A testing method for “drainability of pulp” is defined in JIS P8121 and this method can be classified into “Canadian Standards Drainability Testing Method” and “Schopper Drainability Testing Methods”. In the present invention, the “Canadian Standards Drainability Testing Method” is employed.

The smaller drainability is, the further the beating of fiber progresses and thus the splitting degree of each fiber increases. When the splitting degree of aramid pulp is increased, draining time for papermaking can be adjusted and a paper body with less surface roughness can be obtained, and moreover, the intertwist of fibers which is the frame of the paper body increases, and the surface area of the friction member which is in contact with the mating surface increases and excellent friction property can be obtained.

When only aramid pulp with low drainability is used as aramid pulp forming a paper body, the amount of split fiber is large, and much resin adheres to knots of those fibers. Therefore, flexibility required for a wet paper friction member cannot be obtained, and the contact area with the mating surface decreases, and pore diameter is reduced, and excellent friction property and heat resistance cannot be obtained.

When only aramid pulp with high drainability is used as aramid pulp forming a paper body, draining time in a papermaking process is reduced, and the surface roughness of a paper body tends to increase, and the texture of a wet paper friction member surface becomes rough, and the contact area with the mating surface decreases, and excellent friction property cannot be obtained, and at the same time mechanical strength decreases.

When the amount of aramid pulp forming a paper body the drainability of which is adjusted is below 30 wt % of the total weight of a paper body, a paper body has a porous structure which is formed by fibers mainly made of pulp, and fiber section repeatedly bearing load tends to wear and break easily, and therefore, a wet paper friction member is likely to be worn down or torn off.

When the total amount of aramid pulp forming a paper body the drainability of which is adjusted is above 50 wt % of the total weight of a paper body, the density of fiber increases, the flexibility of a wet paper friction material decreases, the contact area with the mating surface decreases, and excellent friction property cannot be obtained.

In this connection, it is desirable that a friction modifier is blended at 30-60 wt % of a total weight of a paper body. The friction modifier comprises said diatomaceous earth, various graphite particulate matters, activated carbon, and inorganic fiber etc.

In addition, thermosetting resin is impregnated into this paper body at a blending amount of 25-45 wt %, and then the paper body is cured by heating, and formed by pressure. It is desirable that resol-based phenol resin is used for the thermosetting resin.

When surface roughness of a wet friction member is reduced, the number of the contact points as well as the contact area with other surface increases and thus frictional coefficient increases. This is mainly since shearing stress of a boundary lubricating film in a boundary lubricating area is increased due to an increase of the contact area.

For example, high frictional coefficient can be obtained when a hard resin layer on the surface of a wet friction member is removed by a process of surface grinding etc. and the surface roughness is adjusted so that the load length rate Rmr (30%)≧55% as the load length ratio is defined by Rmr(C) (C: cutting level).

On the other hand, when the surface roughness is such that the load length rate Rmr (30%)<55%, satisfactory contact conditions cannot be obtained, nor can high frictional coefficient be obtained.

In this connection, the “load length ratio Rmr” is a surface texture parameter specified by JIS (JIS B0601-2001), which is a ratio of a sum total of base length of convex part of a measured curve to an evaluation length. The convex part extends above plus side relative to a cutting level when the measured curve is cut at a cutting level parallel to an average line. The ratio is shown in percentage (%). The cutting level is shown by the percentage to the maximum height.

As for a method for reducing the surface roughness, there are methods other than said surface grinding, such as forming a paper friction member under the condition of high temperature and using different kind of resin for impregnation.

The present invention will be concretely explained below referring to Example.

Example 1

A wet clutch disk was produced by blending a mixture of 10 wt % of cellulose pulp, 20 wt % of aramid pulp the drainability of which was adjusted to 300 ml, 20 wt % of aramid pulp the drainability of which was adjusted to 600 ml, and 50 wt % of a friction modifier comprising 28 wt % of cylindrical diatomaceous earth the size of which was 1-3 micrometers, and forming a paper body; impregnating it with thermosetting resin at 30 wt %; curing it by heating and then forming it by pressure; bonding it onto a core plate of steel with thermosetting resin; and grinding its surface so that its surface roughness was such that the load length rate Rmr (30%) was 70%.

Comparative Example 1

A wet clutch disk was fabricated by blending a mixture of 36 wt % of cellulose pulp, 7 wt % of aramid pulp the drainability of which was adjusted to 300 ml, 7 wt % of aramid pulp the drainability of which was adjusted to 600 ml, and 50 wt % of a friction modifier comprising 28 wt % of cylindrical diatomaceous earth the size of which was 1-3 micrometers and forming a paper body; impregnating it with thermosetting resin at 30 wt %; curing it by heating and then forming it by pressure; bonding it onto a core plate of steel with thermosetting resin; and grinding its surface so that its surface roughness was such that the load length rate Rmr (30%) was 60%.

Comparative Example 2

A wet clutch disk was produced by blending a mixture of 5 wt % of cellulose pulp, 35 wt % of aramid pulp the drainability of which was adjusted to 300 ml, 35 wt % of aramid pulp the drainability of which was adjusted to 600 ml, and 25 wt % of a friction modifier comprising 28 wt % of cylindrical diatomaceous earth the size of which was 1-3 micrometers and forming a paper body; impregnating it with thermosetting resin at 30 wt %; curing it by heating and then forming it by pressure; bonding it onto a core plate of steel with thermosetting resin; and grinding its surface so that its surface roughness was such that the load length rate Rmr (30%) was 60%.

Comparative Example 3

A wet clutch disk was produced by blending a mixture of 10 wt % of cellulose pulp, 20 wt % of aramid pulp the drainability of which was adjusted to 300 ml, 20 wt % of aramid pulp the drainability of which was adjusted to 600 ml, and 50 wt % of a friction modifier comprising 28 wt % of cylindrical diatomaceous earth the size of which was 1-3 micrometers and forming a paper body; impregnating it with thermosetting resinat 30 wt %; curing it by heating and then forming it by pressure; bonding it onto a core plate of steel with thermosetting resin; and grinding its surface so that its surface roughness was such that the load length rate Rmr (30%) was 40%.

The results of friction property evaluation, heat resistance evaluation, and compressed fatigue strength evaluation of a wet friction clutch disk produced by the above-mentioned process are shown in Table 1. TABLE 1 Comp. Comp. Comp. Example 1 Example 1 Example 2 Example 3 blend cellulose pulp 10 wt % 36 wt %  5 wt % 10 wt % aramid pulp drainability 20 wt %  7 wt % 35 wt % 20 wt % 300 ml drainability 20 wt %  7 wt % 35 wt % 20 wt % 600 ml diatomaceus earth 50 wt % 50 wt % 25 wt % 50 wt % surface roughness Rmr (30%) 70% 60% 60% 40% friction property μd 0.155 0.155 0.129 0.145 revolution: 2940 rmp μs 0.087 0.086 0.075 0.080 surface pressure: μ 0/μd 0.92  0.093 0.94  0.092 0.95 Mpa oil temperature: 100° C. heat resistance μd decline rate 24% 55% 20% 25% calorific value (after 5,000 124 J/cm² cycles) stroking amount of 140 μm    340 μm    120 μm    150 μm    surface pressure: compression 11.8 Mpa (after 200,000 oil temperature: cycles) 120° C.

It is known from the results shown in Table 1 that Example 1 and Comparative Example 1 had high frictional coefficient, but, Comparative Example 1 had short life due to the lack of heat resistance and strength, and on the other hand, Comparative Examples 2 and 3 had sufficient heat resistance and strength, but their frictional coefficients were low.

[ADVANTAGES OF THE INVENTION]

As explained above, according to the present invention, a wet paper friction member with excellent friction property, high frictional coefficient, heat resistance, and compression fatigue strength can be provided by adjusting the blending ratio and drainability of aramid pulp contained in a paper body of a wet paper friction member. 

1 A wet paper friction member which is produced by impregnating a paper body with thermosetting resin, curing it by heating and then forming it by pressure, characterized in that said paper body comprises cellulose pulp and a filler and further comprises aramid pulp the drainability of which is adjusted to 200-400 ml and aramid pulp the drainability of which is adjusted to 450-700 ml, respectively at 10-30 wt % of a total weight of the paper body, and said whole aramid pulp the drainability of which is adjusted comprises 30-50 wt % of a total weight of the paper body: 2 A wet paper friction member as claimed in claim 1, wherein its surface roughness before and after pressure forming is such that the load length ratio Rmr (30%)≧55% as the load length ratio is defined by Rmr (C) (C: cutting level %). 